This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
da7mon12, Nero Wolf, ⎛⎝Y☆MS⎠⎞, iokua41, Person0707, LucasPlays, (George) WolfBite, the_T, shannonmarie098, Sh4dowV0rtex, j_a_c_11_9988, PENVMBRA, alex_meeks14, LembuGhini, joseal_vr11, sa, Deeshy, Squeeze, The War Pug Gaming, [HZ]Pvt. Aaron, gadkareem06, VEXYTOR, trubin-97, SCREAMOguy413, Imperium, piplup_the_panda, [TG] ERROR, rawendhame, Anothink, Pablo Pacman, paloval.11, Arcticblue, zachary-hudson, ajaplaysgames, mj01032003.mj, ferrymeidian, evilgood2010, mariado1574, [HZ]Cpl. Dawson, connorhaytack, Crazy31139, Circonvolutor, Gear2ndGandalf, risoskol, The Aglius V, The Aglius V, MinkyShox, hu6758, Coked Out Cannibal Popstars, Nightwolf +182 others